The present invention relates to force sensors.
More particularly it relates to a force sensor for determination of forces on a vehicle seat, which produces an electrical signal with the use of electrical or electromagnet effect, wherein the thusly produced signal corresponds to a pressing, a pulling or a bending force acting on the force sensor.
Force sensors are required in many mechanical areas and must be provided on relatively unavailable locations of devices and aggregates for an accurate determination of pulling and/or pressing forces. An electrical signal which corresponds to the force measurement must be available for further evaluating or controlling processes. In the automobile electronic systems force sensors are used, which in many instances are relatively large and relatively expensive for manufacturing.
For some important electronic systems in motor vehicles, such as for example for seatbelt restraining systems, a small and compact force sensor is however required. Force measuring screws are of special interest to be provided directly at the connecting points between the components in which the force is concentrated. For example for seat weight force measurement and its distribution which is measured at the connection to the seat, force sensors are needed which must be manufactured in great numbers and inexpensively. However, increasingly accurate, statistically measurement force sensors are required in the manufacturing and quality measuring technique.
Known sensors which have small dimensions are based mainly on the piezo-electric convertor principle and can operate only dynamically. Static force sensors are formed often as bending springs which are provided with strain gauges. These force sensors are very precise, however they are large and expensive.
Magnetoelastic sensors are based, to the contrary, on a crossductor principle which is particularly suitable for applications where at high temperatures it is necessary to operate without electronics and only a small space is available, or in accordance with the torductor principle in which the magnetic field distribution is determined contactlessly depending on force by two U-shaped cores which are turned relative to one another by 90° and provided with windings. The crossductor principle has the disadvantage in that it provides only small nominal voltages with mainly great offset, which are difficult to evaluate. Moreover these sensors can not be used or can be used with high additional expenses for movable or rotatable parts. The torductor principle allows force measurements also in rotatable parts, however, it is very distance-sensitive.
For seat weight sensing, in particular in a motor vehicle, it is necessary that the seat height does not increase and moreover an overload yield is very important. International patent document WO 00/16054 discloses a measuring pickup for movement determination in a vehicle seat, in which an elastic deformation of a support element is determined with a measuring cell, between an upper frame which surrounds the seat shell and a lower frame which is mounted on the vehicle body, in some cases via a longitudinal and high adjusting mechanism.